The invention applies to any type of electrical cable, in particular power transmission cables or communication cables, in fixed or mobile installations. The cables concerned may be coaxial, two-wire, parallel pairs, twisted pairs or other, provided that it is possible to generate, at any point of the cable, a measurable singularity.
A singularity in a cable corresponds to a change in the signal propagation conditions in this cable. Most commonly it results from a fault which locally affects the characteristic impedance of the cable by inducing a discontinuity in its per-unit-length parameters.
To characterize an electrical cable, it is important to know the propagation velocity of a signal in this cable. In particular, an accurate measurement of this velocity is necessary when seeking to diagnose faults on a cable. The fault detection is done, as is known, by injecting a high-frequency signal which is propagated along the cable and is reflected on each of its singularities. From the knowledge of the delay between the various reflections, the faults can be accurately located. However, to convert duration into distance, it is necessary to accurately know the propagation velocity of the signal. The measurement of this velocity must be possible at any instant in order in particular to take account of its variation over time associated with the aging of the cable, or with a modification of the mechanical stresses which can be exerted thereon, such as pulling, twisting or even winding.
Solutions are known for determining the propagation velocity by mathematical and/or physical models and an estimation of the primary parameters of the cable. These solutions suffer from inaccuracy and above all do not take into account the aging of the cable and the trend of the velocity over time.
Other methods, called reflectometry methods, use instruments that are costly, bulky and cannot be carried around, which also suffer from not allowing the measurements to be updated over the life of the cable.
The American patent application US 2010/0045968 describes an alternative method for a non-intrusive measurement of the propagation velocity in a cable as well as of its length from optical beams projected in the transverse direction of the cable and analyzed by two optical sensors.